A graduate of the University of Limerick, Heffernan is a primary school teacher by profession. Before becoming a full-time politician, Heffernan taught in a number of schools, including St Anne's in Whitechapel, London.

He lost the parliamentary Labour Party whip in December 2012 after voting against the government on the Social Welfare Bill.[5] The loss of the whip resulted in a falloff in Heffernan's Seanad voting attendance. In September 2013, it was reported that Heffernan attended only 23% of all votes in the Seanad, the second worst record.[6]

Between July 31 and August 2, 2016, Heffernan was allegedly arrested 3 times for offences including spitting at Gardaí, and drunken behaviour while attending Indiependence Music & Arts Festival in Mitchelstown, Co Cork.[9]